Re: To AC -- on electrons

On Sat, 10 Jun 2006 21:45:46 -0400, Jeffrey Turner
<jturner@xxxxxxxxxxxx> wrote:

r norman wrote:
On Sat, 10 Jun 2006 11:42:08 -0400, Jeffrey Turner
<jturner@xxxxxxxxxxxx> wrote:
John Harshman wrote:


A better way to put this is that we can't perceive individual photons (I
believe it takes around 6 or 7 to produce a visible flash), and so have
no capability to notice that light is quantized.

No, in perfect darkness we can see a single photon. At least
this is what I was taught in physics class some-odd years ago.

The physicist was wrong. My recollection is that it takes perhaps a
dozen photons entering the eyeball and at least two or three
activating receptor cells before we can detect a flash of light. The
original work was
Hecht, S., Shlaer, S., and Pirenne, M.H.:
Energy, Quanta, and Vision,
J. Gen. Physiol., 25: 819, 1942.


See, for example
http://www.princeton.edu/~wbialek/PHY562/060207.pdf

"Sitting quietly in a dark room, we can detect the arrival of
individual photons at our retina."

"The first group to record the single photon responses in
vertebrate rod cells was led by Denis Baylor at Stanford in the
late 1970s. ... Single photon responses observed in this way are
about a picoamp in amplitude vs. a continuous background noise
of 0.1 pA rms, so these are easily detected."

This paper has a graph which shows that the probablity of seeing
a single photon (K=1) can be unity for a high enough light
intensity.

http://www.madsci.org/posts/archives/oct2000/971133010.Ns.r.html

"A dark-adapted rod photoreceptor can actually detect individual
photons -- this is an absolutely spectacular feat of
amplification within the cell!1"

1 S. Hecht, S. Schlaer and M.H. Pirenne, Energy, Quanta
and vision Journal of the Optical Society of America, 38,
196-208 (1942);

Baylor, D.A., Lamb, T.D. & Yau, K.-W. (1979). Responses of
retinal rods to single photons Journal of Physiology 288,
613-634;

Baylor, D.A., Nunn, B.J. & Schnapf, J.L. (1984). The
photocurrent, noise and spectral sensitivity of rods of the
monkey Macaca fascicularis Journal of Physiology 357, 575-607.

Though he also claims that "Under optimum conditions, it has
been found that fewer than 100 photons striking the eye (10 rods
per 1/10 of a second) are enough for rod-mediated vision. 4"

4 David R. Copenhagen and Tom Reuter

Our eyes can certainly detect single photons. Whether we would
be conscious of such obviously requires more in-depth analysis
than these two short papers.


Even the 1942 Hecht, Schlaer, and Pirenne paper I cited acknowledged
that the several photons required for "seeing" were almost certainly
detected by different cells. It is most definitely established that
photoreceptor cells can respond to single photons. The question
remains, though, whether we multicellular humans can "see" and respond
to such cellular events. The psychophysical experiments seem to
indicate that we cannot. Several cellular events must summate to
produce a signal large enough to produce an organismal response we
call "seeing".

I saw the paper you referenced but could find no support in it for its
introductory sentence: ""Sitting quietly in a dark room, we can detect
the arrival of individual photons at our retina." except to mean that
individual cells in our body, not we humans, can do the detection.

.

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